Due to increasingly stringent laws on the maximum allowable amount of volatile organic compounds in coating compositions, major efforts have been made to minimize the use of organic co-solvents in water borne coatings based on acrylic binders. In water borne coating compositions that use dispersions of acrylic polymers as the main binder, the final hardness after curing of an applied coating film is related to the glass transition temperature of the acrylic polymer. A high glass transition temperature of the polymer however means that substantial amounts of organic co-solvent must be used to reduce the minimum film formation temperature in order to ensure proper coalescence of the polymer particles. Since many final coatings require a minimum hardness, for example if a high blocking resistance or scratch resistance is important, and in view of the desire to reduce the amount of volatile organic compounds, improved coating compositions are needed.
It is known that the hardness of a coating can be increased by introducing a cross-linking mechanism in the coating. In coating applications one- (1K) and two-package (2K) systems are commonly used. In order to obtain cross-links in the final coating, various possibilities have been proposed, inter alia depending on the system that is used. More particularly, in a one-package system, the acrylic resins are typically combined with the cross-linker within a single package with curing occurring at ambient temperature or by baking the coating after application. In a two-package (2K) system, the acrylic polymer is typically separated from the cross-linker. Prior to application, the contents of the two packages are combined, and curing occurs through chemical cross-linking reactions.
It should be noted that the conventional polymer compositions for water borne coating compositions are typically supplied as aqueous dispersions. Such aqueous dispersions are conveniently prepared by means of an emulsion polymerization process. Hence, also the polymer compositions according to the invention are preferably aqueous polymer dispersions that are suitably produced using such an emulsion polymerization process. The aqueous polymer dispersions that are being used as binders in these coatings are often prepared by means of the emulsion polymerization process. A general description of the emulsion polymerization process is given E. W. Duck in Encyclopedia of Polymer Science and Technology, 1966, John Wiley & Sons, Inc., Vol 5, p 801-859. A serious drawback to the conventional emulsion polymerization process is the fact that substantial amounts of protective colloids and surfactants must be used. Surfactants are low molecular weight amphiphilic compounds possessing a hydrophilic and a hydrophobic moiety. The amounts used are in general above the critical micelle concentration for the surfactant used. Surfactants perform many functions in emulsion polymerization, including solubilizing hydrophobic monomers, determining the number and the size of the dispersion particles formed, providing dispersion stability as particles grow, and providing dispersion stability during post-polymerization processing. Typical examples of surfactants used in emulsion polymerizations are anionic surfactants, such as fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates, and nonionic surfactants, such as ethoxylated alkylphenol or fatty acids used to improve freeze-thaw and shear stability. Cationic surfactants derived from amines, nitrites, and other nitrogen bases, are rarely used because of incompatibility problems. Often a combination of anionic surfactants or anionic and nonionic surfactants is used to provide improved stability.
Conventional surfactants (also known as emulsifiers) are highly water sensitive and impart poor water resistance to the final coating compositions comprising them. Furthermore surfactants or emulsifiers often act as plasticizer for the polymers resulting in reduced properties such as surface hardness, blocking resistance and chemical resistance. Another problem that might occur is the tendency for surfactant molecules to migrate to the polymer/air or polymer/substrate interface, where deleterious effects (cloudiness at the surface, loss of tack, etc.) are often caused. Recently a number of products have come onto the market, known as “polymerizable surfactants”, where the molecule contains a polymerizable ethylenically unsaturated double bond. An example of the use of such a compound is given in European Patent Application EP-A-1042370. The surfactant becomes bound to the main polymer during the emulsion polymerization. However, it is hard to obtain full conversion of these reactive surfactants. A comprehensive review on this subject is given by Asua et al. (Acta Polym., 1998, 49, 671). The non-converted polymerizable surfactant will behave in a way that is similar to conventional surfactants and hence will also negatively influence the application properties.
An alternative for low molecular weight surfactants is the use of polymeric surfactants. In GB-A-1,107,249 an emulsion polymerization process is described where a styrene/acrylic acid resin prepared by solution polymerization in the presence of a chain transfer agent, having an acid value of about 169 and a molecular weight of about 2100, was used, after neutralization, as a polymeric surfactant for an emulsion polymerization. The drawback of polymer dispersions thus made is that the polymeric stabilizer is not covalently bound to the polymer dispersion particles. As a result the resistance against water and alkaline solutions of the final coating is very poor. A solution to this problem is known from PCT patent application WO-A-95/29944. This patent application discloses a process for the production of an aqueous polymer composition for use as a coating with improved hardness and low film forming temperature. Use is made of a cross-linking agent that links a first, water-soluble polymer with a second, hydrophobic polymer. The cross-linking agent reacts by condensation. The lower the water concentration, the more this reaction takes place. It was found that the cross-linking reaction proceeds relatively slowly and begins only after evaporation of all the water from the film. As a result, the early hardness and water resistance of the final coating are relatively low.
European patent application EP-A-0,587,333 discloses a water-resistant polymer dispersion containing a carboxylated water-soluble polymer that is neutralized and solubilized with, e.g., ammonia. An alkali-insoluble emulsion polymer was prepared in the presence of the water-soluble first polymer. During preparation, the two polymers are grafted together. The alkali-insoluble polymer can have amine functionality. The composition can comprise a water-soluble polymer with an amine functionality, which serves to neutralize the soluble polymer, as an alternative for the ammonia. Metal ions may be incorporated into the monomer mixture so as to create ionic metal/carboxylate cross-links. The water-soluble stage polymer remains susceptible to bases, which may cause resolubilization when a second layer of paint is applied. The hydrophobic polymer remains thermoplastic and does not cross-link, so that the resulting coating film shows a limited hardness after application. A common problem with aqueous polymer compositions comprising polymer dispersions with polymers having substantial amounts of carboxylic acid-functional groups, is their poor water resistance and poor re-coatability, since the water-soluble polymer will easily be affected in the presence of water or, when a second layer of water borne paint is applied, under the influence of the neutralizing base present. Also the resulting coatings suffer from a bad chemical resistance.
In European patent application EP-A-0,989,163 a solution is proposed to solve these problems. The water-soluble first stage polymer has carbonyl functionality, so that it can be cross-linked by means of a co-reactive compound. Additionally, an amine-functional water-soluble third polymer was included in the composition to form ionic cross-links with the acid-functional groups in the polymer. However, even though the polymer dispersions described in this patent give significantly improved properties compared to the prior art, some properties still are inadequate. It was found that these deficiencies are caused because only about 50% of the water-soluble first stage polymer was permanently attached to the substantially water-insoluble second polymer. The water-soluble polymer can be permanently attached by entanglement or by chemical grafting through proton abstraction. The degree of attachment can be determined by means of techniques such as ultracentrifugation of the polymer dispersion and analysis of the solids content of the supernatant.
It is to be expected that the degree of attachment of the water-soluble polymer will be increased by the introduction of a co-polymerizable ethylenical unsaturation at one end of the polymer chain. Hence, unsaturated chain transfer agents such as mercapto-olefins may be used, as these will introduce unsaturation into the oligomer.
Alternatively, various living polymerization methods have been disclosed for obtaining functional-ended polymers by selective termination of living ends. Such functional-ended polymers can subsequently be attached to another polymer, that is, as so-called macromonomer “arms” on a polymeric backbone to form a comb graft copolymer. Webster, in Living Polymerization Methods, 251 Science 887 (22, Feb. 1991) generally discloses living polymerization methods for preparing architectural forms of polymers, including graft and comb copolymers. Living polymerization methods suffer from a lot of disadvantages, not the least being the susceptibility to contaminants.
An alternative for living polymerization methods in the production of macromonomers was described in U.S. Pat. No. 5,710,227 wherein a high temperature polymerization process was used for making terminally unsaturated oligomers. This polymerization has to be conducted at above 225° C. to produce polymers having a degree of polymerization that is sufficiently low. This is not only undesirably energy intensive, but also causes degradation of some functional monomers that are needed for cross-linking. In patent application WO-A-00/05272 the macromonomers described above are used as polymeric stabilizers in an emulsion polymerization process. However also in this case, conventional low molecular weight surfactant has to be used in the polymerization recipe to control particle size.
U.S. Pat. Nos. 4,680,352 and 4,722,984 disclose the use of cobalt (Co) chelates as chain transfer agents in free-radical polymerization. The latter patent discloses that macromonomers prepared by cobalt chain transfer can be polymerized to produce graft copolymers which are useful in coatings. In U.S. Pat. No. 6,017,992 a process for the production of an aqueous polymer emulsion is described which comprises the preparation of a low molecular weight polymer containing acid-functional groups using a free-radical polymerization process which employs a free-radical initiator and a transition metal chelate complex, particularly a cobalt chelate complex. This mode of operation produces polymers with number average molecular weights within the range of 500 to 50,000 and a terminal α-substituted acrylate group. A drawback to this process is that the end product contains cobalt, which is undesirable from an environmental point of view. Furthermore conventional low molecular weight surfactant is needed to control particle size.
It is an object of the present invention to find specific polymer compositions, which can be used as a binder in water borne coating compositions with a low content of volatile organic compounds, a process to make them that is devoid of the above disadvantages, and coating compositions comprising said polymer compositions that show good film formation, fast hardness development, and good chemical resistance.